Digital electronics have made it possible to record a grey scale or color image of a scene, as a still image, as a series of still images, or as a video. A video is a series of still images that continues for an extended period of time with a specific interval between each image. Analog imaging utilizes photographic film to obtain an image, whereas digital imaging utilizes a focal plane array (FPA) to obtain an image which provides a signal in response to light illumination that is then digitized. The FPA includes an array of light-detecting elements, or pixels, positioned at a focal plane of optics that image a scene. Much recent effort has been directed to improving the density, size, sensitivity, dynamic range, and noise characteristics of FPAs, as well as the associated optics and electronics, enabling higher resolution images to be acquired. However, most FPAs by their nature cannot detect color, only the presence and quantity of light. Additional techniques have been developed to recreate the color seen by the human eye in a color digital image, such as the use of Bayer filters as described in U.S. Pat. No. 3,971,065, and subsequent developments thereof, or multiple FPAs with bandpass color filters. Other FPAs have been developed that detect color directly.
Additionally, FPAs are limited to collecting information about light emanating from a scene in two dimensions, horizontal (x) and vertical (y), in front of the imaging device, often referred to as the field-of-view (FOV). Most FPAs cannot, by themselves, obtain information about the distance (z) of an object from the FPA without the use of complex, high speed, expensive read-out circuits. A wide variety of imaging techniques have been developed to attempt to extract, from a two-dimensional image, information about the distance of a scene and of three-dimensional objects within that scene. Some such techniques may be based on information in a single two-dimensional image, such as analyzing the positions and depths of any shadows and the apparent position and type of light source to infer information about the distance of objects in the image. Other such techniques, often referred to stereoscopy or stereo photogrammetry, may be based on obtaining multiple two-dimensional images with multiple cameras positioned at different positions relative to the scene, and comparing information within the images to deduce the ranges and three-dimensional features of objects within the scene. Both types of techniques typically are computational intensive, provide only limited information about the three dimensional features of a scene, and may be unsuitable for moving objects. Additionally, stereoscopy typically requires precise knowledge of the relative position and angle at which the multiple two-dimensional images are obtained and so requires extensive calibration procedures and limited flexibility. The multiple views also means that more lines of sight will be obscured. This limits the use of such system in uncontrolled environments, can significantly increase the cost of any implementation, and limits the accuracy and precision of any calculated distance values.
Another approach to obtaining distance information for objects in a scene is based on scanning a laser beam over the scene, and determining the ranges and three-dimensional shapes of objects in a scene based on a phase or temporal delay of the laser beam, following reflection from the object. Specifically, the distance the laser beam travels from the light source, to a particular point in the scene, and then to a sensor can be calculated based on the phase delay or time of flight (TOF) of the laser beam, and the speed of light. Distance and shape information about objects in the scene may be obtained by scanning the laser beam, one point at a time, across the entire scene, and determining the phase delay or TOF of the laser beam at each point. Such scanning may be accomplished, for example, by moving mirrors or beam steering elements to change the beam direction. As such, the maximum scanning speed may be limited by the amount of time required to make a measurement at each point, and the speed of the mirror or beam steering element. Some such laser scanners are limited to processing tens of thousands to hundreds of thousands of points per second. Therefore, obtaining a high resolution image of a complex scene may take a large amount of time, although lowering the resolution of the image may reduce the time required to obtain the image. Image quality also may be degraded by performance drift during the scan, or motion within the scene. Additionally, scanning merely provides the value of the distance at each measurement point, resulting in what may be referred to as a “point cloud;” often no color or intensity information is obtained, and additional steps are required to transform the point cloud into a digital representation more suited to human interpretation. For example, color or grey-scale imagery may be collected in a separate step and combined with the point cloud data if a complete 3-dimensional image is desired.
U.S. Pat. No. 5,157,451 to Taboada et al. (“Taboada”), the entire contents of which are incorporated herein by reference, describes an alternative technique that combines digital imaging with distance measurements for long-range imaging of target objects. Specifically, Taboada discloses obtaining three-dimensional coordinates of a target object by irradiating the object with a laser pulse, and using a Kerr cell or Pockels cell to vary the polarization of the laser pulse reflected from the object as a function of time. As a result, the polarization state of portions of the laser pulse reflected by features of the object nearer the imaging system (shorter TOF), is affected to a small degree, while the polarization state of portions of the laser pulse reflected by features of the object further from the imaging system (longer TOF), will be affected more. By imaging the two polarization components of the polarization-modulated laser beam onto two separate FPAs, positional information about the object may be calculated. However, the systems and methods disclosed by Taboada have limited applicability, some of which are discussed further below.
As noted above, the system of Taboada utilizes a Kerr cell or Pockels cell, which are particular types of electro-optic modulators (EOMs), to modulate the polarization of the reflected laser pulse. In an EOM, an electric field is applied to a material that changes properties under the influence of an electric field. The EOM's change in properties modifies the phase of light transmitted therethrough. Pockels cells are based on the Pockels effect, in which a material's refractive index changes linearly with applied electric field, while Kerr cells are based on the Kerr effect, in which a material's refractive index varies quadratically with the electric field. For certain materials and certain orientations of applied electric field, the Pockels effect creates an anisotropy in the refractive index of the material. Such materials and fields may be used to create a Pockels cell, in which the induced anisotropy changes the polarization state of light transmitted therethrough linearly as a function of applied voltage. EOMs such as Pockels cells may be placed between crossed polarizers to modulate the intensity of light, as is known to those of ordinary skill in the art. The temporal response of a Pockels cell may in some circumstances be less than 1 nanosecond, enabling its use as a fast optical shutter.
Although widely used for laser applications, Pockels cells traditionally have been viewed as having significant limitations, rendering such devices unsuitable for optical switching in other types of applications. For example, in some applications, the incident light may contain a large range of angles. However, typical Pockels cells may only effectively modulate incident light deviating by less than about 1 degree from the surface normal, significantly limiting their use in such applications. Additionally, Pockels cells may require high electric fields, e.g., in excess of several kilovolts, to sufficiently rotate the polarization of light passing therethrough. The electronics required to generate such fields may be expensive and cumbersome. One approach for reducing the voltage required to drive the Pockels cell has been to use a transverse electric field and a transversely oriented Pockels cell. The phase change induced in such a cell is proportional to the ratio of the crystal thickness d (which is also the separation between the electrodes) to the crystal length L as given by:
                    V        ∝                              λ            ⁢                                                  ⁢            d                                2            ⁢                          n              3                        ⁢                          r              ij                        ⁢            L                                              (        1        )            where V1/2 is the half-wave voltage, i.e., the voltage required to induce a phase delay of π in light of one polarization relative to orthogonally polarized light, λ, is the wavelength of light, n is the refractive index of the crystal, and rij is the electro-optic tensor coefficient of the crystal. Reducing the thickness of the electro-optic crystal to bring the electrodes closer together may reduce the voltage, but also may reduce the clear aperture of the Pockels cell and may cause vignetting, e.g., loss of information at the edges of the image, reducing image quality. New materials are being sought that may function satisfactorily at lower voltages, such as periodically poled lithium niobate.